The actual position and velocity of a photoreceptor belt is accurately measured by sensing the passing of illuminated holes in the belt, as the belt moves in a process direction. A segmented sensor array is positioned so as to view the passing of the illuminated holes. A Gaussian light intensity distribution is sensed by a group of sensor pixels which sense the light during a sampling interval. The array produces an output which is operated upon by a centroid processor to determine the center of moment for each sampled intensity distribution. A prediction is then made for projected position of the belt which is very accurate since the centroid calculation is not effected by noise produced by stray light or poorly defined image edges.